Simulcast paging systems convey data or voice signals from a central terminal to a plurality of paging transmission sites, which then modulate the data for transmission to pager receivers carried by subscribers to the service. The paging transmission sites thus each include an exciter that modulates an input signal and produces an output signal in a desired modulation format for amplification and transmission over a paging zone. The specific type of modulation used depends upon the requirements of the simulcast system and may include standard frequency modulation (FM), frequency shift keying (FSK), four level FSK, variants of .pi./4 quadrature phase shift keying (QPSK), variants of quadrature amplitude modulation (QAM), variants of amplitude companded single side band (ACSSB), and digital voice compression. The modulation scheme may also have to meet the requirements of the relatively new standard for high-speed paging systems in Europe (ERMES). In addition, the modulator must be sufficiently versatile to operate with different types of input signals, including analog voice and paging data at rates up to, and possibly in excess of 2400 baud, and with an input signal that may contain DC levels. Although different hardware modulators can be designed to handle each of the various input signal and modulation formats, it would clearly be more cost effective to provide a digital modulator that can readily be changed by modifying the software that controls its operation, to handle any of these requirements.
The advantages of a paging system modulator based upon a digital signal processor (DSP) have already been recognized in the art. Such a device is described in a paper entitled "4-PAM/FM Modulator with DSP: A Solution for ERMES," by Jaime Bustillo, Miquel Rodriguesz-Palanca, and Javier Perez, presented at the May 1991 Vehicular Technology Conference. The paper notes that the frequency specifications of the proposed ERMES standard are difficult to meet, particularly, the specification for a center frequency stability of .+-.15 Hz and a difference between any two adjacent symbol frequencies of 3,125.+-.15 Hz. However, use of the DSP for a signal modulator, as disclosed in the paper, enables these requirements to be met. The disclosed DSP implements data preprocessing, includes an integrator, and uses a transition table to store the possible transitions between four possible different data symbols (four level FSK). The data are convened to complex values by multiplying with sine and cosine values stored in lookup tables. The resulting products are then converted from digital-to-analog (D-A) format and respectively multiplied by quadrature RF signals in an analog quadrature modulator before being added together for transmission as an RF output signal.
The solution to the problem disclosed in the above referenced paper is only a partial one. The approach used in this prior art DSP tends to use excessive power and requires a relatively expensive high-speed D-A converter, since the typical RF frequency of the output signal is usually in the tens of MHz range. Since the disclosed prior art DSP modulator discussed above is designed to modulate only a digital signal, the paper describing it does not include any component to eliminate spurious noise artifacts that result from sampling an analog signal at a different and usually lower rate than that at which a quadrature modulator operates. Modulation of the input signal (prior to its quadrature modulation) should include some means for carrier frequency adjustment, deviation limiting and adjustment, and intermediate frequency (IF) filtering--none of which are disclosed in the above referenced paper.
Use of a sine and cosine lookup table to produce a complex signal conversion in the modulator typically produces excessive spurious noise unless sufficient resolution is provided in the table. However, to reduce the spurious noise to the desired -90 dB level would require a lookup table of about 64 Kbytes in length--generally much larger than desirable in a low-cost modulator.
If the input signal to the modulator comprises a data signal that is asynchronously sampled at a relatively low rate of, for example, 19,200 Hz rate by a DSP modulator, there is a significant jitter problem, because the transitions between logic levels of the data signal occur asynchronously at points in time that may differ from the times at which the samples occur. In most instances, a maximum jitter rate of .+-.1 .mu.s is desired, but to obtain this precision in the sampled signal would require an unacceptable increase in the sample/processing rate.
Conventional modulators provide analog adjustments to control characteristic parameters of the modulation scheme employed, including parameters such as the deviation frequency for frequency shift key modulation, the center frequency of the modulation, an analog deviation limit, an analog frequency deviation level, and an FSK frequency deviation level. These adjustments are typically carried out by "tweaking" a variable potentiometer, variable inductor, variable capacitor, or other analog device, while monitoring the effect of the adjustment by measurements made on test points in the circuit with specialized test equipment. Occasionally, setting the adjustment may require checking the modulated RF signal picked up on a receiver. These adjustments are susceptible to imprecision in setting a parameter, and the values to which the parameters are adjusted are likely to drift over time due to aging of components or due to changes in ambient conditions, such as temperature. After adjustment of some of the parameters, it may be necessary to allow the modulator to stabilize for 20-30 minutes before the effect of an adjustment is correctly determined. Even a prior art digital signal modulator, such as that disclosed in the Bustillo et al. paper noted above, does not apparently include any provision for digitally defining these parameters or for controlling the modulation of an RF signal using digital values for the parameters that are input by an operator. Clearly, such a feature would provide a significant advantage in setting up a modulator used in a paging system or for other purposes. It would also be beneficial to be able to set these parameters from a remote location, enabling a technician, for example, to remotely adjust the offset in the center frequency to different values for modulators in each of the simulcast transmitters having overlapping zones.
If the digital adjustment and control of the modulator provides a desirable advantage, a related improvement over current technology would be the provision for displaying the values of certain operating conditions in the modulator in digital format. For instance, signals such as a modulation detection signal (indicating that an input signal is being provided and that a modulated signal is being produced), a frequency deviation signal for FSK modulation, an analog frequency level signal, and a center frequency offset signal should be available to an operator, both at the site of the modulator and from a remote location to enable monitoring the operation of the modulation process. The prior an does not provide any such capability integral to the modulator; instead, additional instrumentation must be provided to monitor these conditions.
The above discussion identifies several problems that should be resolved to provide a more versatile digital modulator that is programmable, so that it can be used to modulate different kinds of input signals to produce selected output signal formats. The advantages and features of the present invention, with regard to the above-noted problems, will be apparent from the attached drawings and the Detailed Description of the Preferred Embodiments that follows.